Caption: Mechanism of male sexual arousal. Computer artwork of the human male reproductive system showing the penis before (flaccid, centre) and during (erect, upper left) sexual arousal. The testes (testicles) can be seen (purple, egg-shaped) connected by the vas deferens (tubes) to the ejaculatory ducts (with the seminal vesicles situated either side) and then the prostate gland (heart-shaped). When aroused, the parasympathetic nervous system stimulates the trabecular arteries (not seen) to dilate, filling the penis tissue (corpora spongiosum, red) with blood. This causes the penis to become hard and erect.